Digitally Adjustable Dancing Lights

By Abhishek Kumar Singh

7255 You might have come across several types of adjustable dancing lights (flickering LEDs). Most of them use presets (variable resistors) to adjust the rate of switching. Being a mechanical component, the preset easily wear out with use and also introduces noise in the circuit. The adjustable dancing lights circuit presented here selects different values of resistors to control the frequency of an astable multivibrator using timer IC 555.

Adjustable Dancing Lights Circuit

The circuit is built around decade counter IC 4017, a quad bilateral switch IC 4066 (to select the desired resistance) and timer 555. The decade counter output selects one of the resistors at the output of IC 4066. The selected resistor changes the time period of the 555 timer circuit, whose ‘on’ and ‘off’ timings are given by:

• ton = 0.693(R9+Rx)C3
• toff = 0.693RxC3

In the present design, R9 = 1 kilo-ohm, C3 = 47 µF and Rx varies according to the output of IC1 (CD4017), which changes by pressing the switch S2 one by one.

When
Rx=R5=5.6 kilo-ohms, ton=0.214969 second, toff=0.182398 second
Rx=R6=10 kilo-ohms, ton=0.358281 second, toff=0.325710 second
Rx=R7=22 kilo-ohms, ton=0.749133 second, toff =0.716562 second
Rx=R8=33 kilo-ohms, ton=1.107414 second, toff=1.074843 second